I. Field of the Invention
The present invention relates to fluid treatment systems and compositions comprising carboxylic acid functional and sulfonyl and/or sulfonate functional polymer(s) and scale control agent(s), and methods for stabilizing metal ions in aqueous compositions, such as hydraulic fracturing compositions, using the fluid treatment systems and compositions.
II. Technical Considerations
Scaling is the precipitation of a salt from a solution that is supersaturated with respect to the salt. The salts include but are not limited to salts of calcium, magnesium, barium, strontium, iron, aluminum, manganese, and so on. Common scales include but are not limited to barium carbonate, barium sulfate, calcium carbonate, calcium sulfate, calcium phosphate, calcium silicate, iron carbonate, iron hydroxide/oxide, magnesium silicate, silica, and strontium sulfate. In brackish water, sodium chloride can even precipitate.
The potential to scale, the rate of scale formation, and the crystal structure or lack thereof are influenced by factors such as the concentration of ions comprising the scale, the nature and concentration of electrolytes in solution, the temperature, residence time, system cleanliness, and presence of additives. Mathematically, when the product of ion concentrations, each raised to a power equal to its formula coefficient, exceed the solubility product constant, the solution is supersaturated. The solubility constant is temperature dependent. Many scales have inverse solubility, i.e., the higher the temperature the lower the solubility (hence solubility product constant). The concentration of electrolytes in solution also affects solubility and calculations of saturation must be corrected accordingly. Dirty systems provide seeding of crystals and therefore scaling will occur more quickly in dirty systems than in clean systems.
As mentioned, residence time impacts scaling. Many inhibitors work by adsorbing onto crystallite surfaces, thus retarding further growth and favoring re-dissolution of crystallite ions. When crystals do form, growth will be modified and less adherent. Inhibitors affect the kinetics of growth and therefore only delay growth. Given enough time, crystals will form. However, for practical purposes, proper treatment can control scaling under the right conditions. That being said, the longer the residence time, the more likely scaling will occur.
Threshold inhibitors work on a sub-stoichiometric basis, meaning at levels substantially below a molar ratio of inhibitor to ion. Chelants work on a stoichiometric basis, meaning one mole of chelant is needed per mole of ion. Thus lower doses of threshold inhibitors are typically required than of chelants.
A useful index for assessing the scaling potential of a system for calcium carbonate is the Langelier Scale Index (LSI). This index models the impact of a combination of alkalinity, calcium ion concentration, total dissolved solids (TDS), pH, and temperature of water for the potential of calcium carbonate scale formation. More specifically, the LSI is an equilibrium model derived from the theoretical concept of saturation and provides an indicator of the degree of saturation of water with respect to calcium carbonate. It can be shown that the LSI approximates the base 10 logarithm of the calcite saturation level. The Langelier saturation level approaches the concept of saturation using pH as a main variable. The LSI can be interpreted as the pH change required to bring water to equilibrium. In order to calculate the LSI, it is necessary to know the alkalinity (mg/L as CaCO3), the calcium hardness (mg/L Ca2+ as CaCO3), the total dissolved solids (mg/L), the actual pH, and the temperature of the water (° C.).
Solubility product concentrations are exceeded for various reasons, such as partial evaporation of the water phase, an increase in pH, or temperature, and the introduction of additional cations or anions. Ion concentrations in excess of the solubility product will tend to promote precipitation of insoluble compounds. For example, mine pool water that is pumped to the surface undergoes degassing of CO2 followed by an increase in solution pH. The corresponding LSI shifts from a negative value (corrosive) to a positive value (scaling). LSI is often used by water treatment specialists to describe the scaling potential of a water for applications such as, for example, in cooling towers.
As the various reaction products precipitate on surfaces of the water carrying system, they form scale or deposits. This accumulation prevents effective heat transfer, interferes with fluid flow, facilitates corrosive processes, and harbors bacteria. In piping and tubing, scale can cause restriction to flow and high friction loss. This scale is an expensive problem causing delays and shutdowns for cleaning and removal.
The presence of iron provides a significant and complex problem in well stimulation operations. Ferrous iron under down-hole conditions can form an iron carbonate scale know as siderite. Iron in the ferric state can form iron complexes that can block flow pathways and inhibit the flow of gas and/or oil therethrough. Also, iron can impair the performance of fracturing fluid components, such as the friction reducing additive.
In oil and gas wells, air can be introduced into water present in the underground formation through the borehole or from comingling of underground water with air-saturated water which has been pumped from the surface into the well. Ground or well water typically exists in a reducing environment. As a result, iron in ground or well water typically is present as the ferrous ion (Fe+2) species. The ferrous iron can originate from many sources, such as the minerals contained within stratigraphic formations surrounding the water or from additives added to the water during oil or gas well drilling or fracturing operations. Exposure to air (oxygen) or other oxidants (chlorine, bromine, stabilized bromine, etc.) causes ferrous ions to be oxidized to insoluble ferric (Fe+3) ion complexes. Ferric ion complexes, such as hydrated ferric oxides (Fe2O3.nH2O), are much less soluble than ferrous iron, and once formed can readily precipitate. The accumulation of these solids can block pores and flow pathways (or fracture conductivity) in the oil or gas well formation, thus causing permeability impairment with an associated decline in oil or gas flow. While not intending to be bound by any theory, it is believed that when iron is present in soluble or dispersed form, it is less likely to block the flow pathways, thus enhancing production potential of the well.
The formation or precipitation of iron oxides can be inhibited by stabilization of the ferrous ion, and/or suspension or dispersion of the iron oxide(s). Stabilization is the process by which polymers: (1) form stable complexes with dissolved iron, thus preventing the formation of insoluble Fe2O3.nH2O and (2) absorb onto the surface of particulates that are forming, thereby greatly restricting particle growth and thus allowing the particles to remain suspended. In contrast, dispersion is the process by which pre-formed iron oxide (Fe2O3) particles are prevented from settling by the action of a polymer. Dispersants are generally negatively charged, low molecular weight polymers. Likewise, the surface charge of iron oxide particles is negative. The repulsion between the negatively charged particle surface and negatively charged polymers prevents the particles from agglomerating and settling.
To prevent clogging of the flow pathways in oil and gas well formations, chelating agents have been used. Citric acid, ethylenediaminetetraacetic acid (EDTA) and nitriloacetic acid (NTA) are common iron chelating agents used for iron control in fracturing fluid design. Chelating agents function on a stoichiometric basis, i.e., one mole of chelating agent is needed per mole of iron. Additional chelating agent is needed to drive the reaction, with the dose depending on the conditional stability constant (K=[complex]/[metal][chelating agent], K being a function of pH). Thus, high doses of chelating agent are needed. The large dose requirement of citric acid results in pH depression, which in turn can negatively impact some friction reducing additives, such as polyacrylamide-based products. While sulfonated polymers have been used to disperse pre-formed ferric iron particulates and/or to stabilize low levels (≦10 mg/L) of ferrous ions in cooling water applications, they have not been used in oil and gas well water to stabilize the high levels of ferrous ions and/or ferric oxide particulates which can exceed 25 mg/L.
In another aspect of the stimulation process, during the hydraulic fracturing operation fluid is pumped at high velocity and high pressure drops are encountered, resulting in large energy losses. Pressures at the surface of the well of 3,000 to 15,000 psi are often required to overcome the frictional losses and fracture initiation pressure. It is well known that energy is lost due to frictional forces encountered during the movement of liquid through a pipe, tubing or conduit. The energy loss is reflected in a progressive drop in pressure measured along the path between the inlet and discharge point. Factors such as fluid velocity, pipe diameter, pipe length, interior surface roughness, fluid density, and fluid viscosity impact the pressure drop, also known as differential pressure.
Well-known laws of fluid dynamics correlate pressure drop as being proportional to fluid velocity. The Reynolds' number (Re) is a dimensionless number that gives a measure of the ratio of inertial forces (ρV2L2) to viscous forces (μVL), and is used to describe different flow regimes, such as laminar or turbulent flow: laminar flow occurs at low Reynolds numbers, where viscous forces are dominant, and is characterized by smooth, constant fluid motion, while turbulent flow occurs at high Reynolds numbers and is dominated by inertial forces, which tend to produce random eddies, vortices and other flow instabilities. As fluid velocity increases, the conditions change from laminar to transitional to turbulent flow. Under laminar conditions flow is smooth and energy loss is minimal, while under turbulent conditions random impurities and other flow instabilities contribute to greater energy loss. Generally, turbulent flow exists when the Reynolds' number of a fluid is >5,000. For the most part, hydraulic fracturing operations occur in the turbulent flow regime. Therefore, reducing energy loss results in significant economic and safety incentives based on lower operating pressures, less equipment fatigue, lower horsepower demand and less capital for equipment.
It is well known that small amounts of high molecular weight polymers can be very effective in reducing friction loss of flowing aqueous fluids. Slickwater applications have been effectively applied in the hydraulic fracturing of Barnett Shale and other unconventional gas shale applications. Certain metal ions, such as ferrous iron, are known to degrade polyacrylamide polymers. The exact mechanism for this degradation is not completely understood but is thought to proceed by a free radical mechanism. Since oxygen is known to accelerate degradation, it seems plausible that an oxygen-anion radical is formed when a metal ion is oxidized. The highly reactive oxygen-anion radical then can attack the polymer chain, scission the polymer backbone and result in performance deterioration.
Also, carbonate and sulfate ions can be present in flowback water from fracturing operations. The fracturing fluid in the down-hole environment can release soluble salts from the formation that can combine with the fracturing fluid and form precipitates such as calcium carbonate, calcium sulfate, barium sulfate, strontium sulfate, and iron carbonate within the underground fracture network and cause scale accumulation in perforations or fissures in the fractured rock
There is a long-felt need in the art for alternative metal ion stabilizers that can be used to control high levels of ferrous ions and/or ferric oxide particulates which can exceed 25 mg/L in aqueous solutions, such as are typically found in hydraulic fracturing applications, can inhibit scale formation, and which are compatible with or provide enhanced performance of friction reducing agents. A metal ion stabilizer or a metal precipitant dispersant that would mitigate the adverse impact of metal ions, such as ferrous iron or calcium or magnesium, on the friction reduction additive would be of significant advantage to the well drilling industry.